Solid waste collected from residential and commercial sources can be used as processed refuse fuel (PRF) in a power boiler or an incinerator operation such waste being preliminarily treated by magnetic separation and shredding operations. The burning of this fuel produces (1) fly ash commonly collected in known manner from the products of combustion stream and (2), solid residue in the form of a generally granular, random sized, free flowing bottom ash. This ash residue contains, inter alia, ferrous and non-ferrous metals and materials suitable for use as a lightweight aggregate substitute in making concrete and like products of the construction industry. Thus there is significant economic advantage to be derived from recovery of such materials from the ash residue. Additionally, the non-metallic recoverables advantageously can be used as a diluent of marginally hazardous fly ash so that same can be disposed of in a conventional landfill disposal operation.
In developing a practical recovery system for handling bottom ash, applicants herein have employed such a system wherein the bottom ash was subjected to first and second screening operations to obtain various fractions of material including a gravel-like aggregate fraction from the ash, such screening operations being intervened by a magnetic separation operation wherein ferrous materials were separated out from a non-aggregate residue fraction. That non-aggregate residue fraction was treated in the second screening operation to yield a large size objects fraction with the remainder fraction being milled and then subjected to a third screening operation to recover as separate fractions non-ferrous materials and further aggregate material. That system functioned satisfactorily but it was discovered that in a number of areas it was desirable that the system be improved by employing measures to:
1. More effectively lessen the presence of ferrous materials in the aggregate recovered.
2. Avoid interruption in the primary screening operation caused most usually by clogging of the screen units with wire and like material present in the bottom ash.
3. Provide greater flexibility in conveniently shifting system operation from a primarily aggregate recovery mode to a subsidiary mode, e.g., one for diluting fly ash with aggregate so same can be disposed safely in a landfill site.
4. More speedily eliminate presence of large or outsize objects (objects 4 inches or more in size in at least one dimension) in the system.
5. Reduce the level of direct labor involvement in the system operation particularly as an incident of indirect delivery of bottom ash from the boiler to the recovery system.